There have been proposed a number of photoelectromotive force members having a photoelectric conversion layer composed of amorphous silicon for use in photovoltaic devices and the like.
Various methods for preparing photoelectric conversion layers by means of vacuum evaporation, thermal induced chemical vapor deposition, plasma chemical vapor deposition, reactive sputtering, ion plating and light induced chemical vapor deposition have also been proposed.
Among those methods, the method of thermal induced chemical vapor deposition (hereinafter referred to as "CVD method") had once been frequently used in various applications. However, such methods are not usually employed for the reason that, besides requiring an elevated temperature, a practically usable photoelectric conversion layer cannot be obtained as expected.
On the other hand, the plasma chemical vapor deposition method (hereinafter referred to as "plasma CVD method") has been generally evaluated as being the most preferred and is currently used to prepare the photoelectric conversion layer of photoelectromotive force members on a commercial basis.
However, for any of the known photoelectromotive force members which have a photoelectric conversion layer composed of amorphous silicon, even if it is obtained by the plasma CVD method, there are still unsolved problems regarding performance characteristics, particularly its electric and optical characteristics, deterioration resistance, and use-environment characteristics upon repeated use, and also homogeneity, reproducibility, and mass-productivity.
Now, although the plasma CVD method is widely used nowadays as above mentioned, that method is problematical due to the fact that it is practiced under elevated temperature conditions and other problems are associated with the apparatus to be used.
Regarding the former problems, because the plasma CVD method is practiced while maintaining a substrate at an elevated temperature, firstly the kind of a substrate to be used is limited to one that does not contain a material such as a heavy metal which can migrate and cause changes in the characteristics of a deposited layer to be formed and secondly, its thickness is likely to be varied, whereby the resulting layer lacks uniformity of thickness and homogeneity of the composition, which may itself also cause changes in the characteristics of the layer to be formed.
Regarding the latter problems, the operating conditions employed with the plasma CVD method are much more complicated than the known CVD method, and are extremely difficult to be generalized.
That is, there already exist a number of variations even in the corelated parameters of substrate temperature, the amount and the flow rate of gases to be introduced, the pressure, the high frequency power for forming a layer, the structure of the electrodes, the structure of the reaction chamber, the exhaust rate, the plasma generation system, etc. Under these circumstances, in order to prepare a desirable photoelectric conversion layer for a photoelectromotive force member, it is required to choose precise parameters from a great number of varied parameters. And there sometimes occurs a serious problem that because of the precisely chosen parameters, the plasma may attain an unstable state which often imparts unexpected troublesome effects to a photoelectric conversion layer to be formed.
As for the plasma CVD apparatus, its structure becomes complicated because the parameters to be employed are precisely chosen as above stated, and whenever the scale or the kind of the apparatus to be used is modified or changed, the apparatus must be so structured as to still provide said precisely chosen parameters.
In this regard, even if a desirable photoelectric conversion layer should be fortunately produced, the photoelectromotive force member product will become costly for the reasons that a heavy investment is necessitated to set up a particularly appropriate apparatus therefor. Then, as there still exist a number of process operation parameters even for such an apparatus, the relevant parameters must be precisely chosen from the existing various parameters for the preparation of such a desirable photoelectric conversion layer, and thus the process must be carefully practiced.
Against this background, there is now an increased demand for providing a method that makes it possible to practice the process at lower temperature and at a high film forming rate in a simple apparatus to mass-produce a desirable photoelectric conversion layer for a photoelectromotive force member which has good uniformity and has practically applicable characteristics.
Besides silicon, there is a similar situation with respect to other kinds of photoelectric conversion layers for photoelectromotive force members such as silicon nitride, silicon carbide, and silicon oxide layers.